1. Field of the Invention
The present invention relates to a water filtration process and an apparatus thereof and more particularly, to a multistage process and a package apparatus thereof for removal of dissolved, colloidal, suspended, volatile, and living contaminants from water.
2. Description of the Prior Art
Various types of filtration processes are well known in the art. In such filtration processes, the suspended contaminants in water are commonly removed by a physical separation procedure wherein the mixture of water and suspended contaminants are forced by gravity force or pumping force through a porous filtration barrier.
Such filtration barrier also known as a filter medium may be fabric of heavy weave, a micro scale wire screen, a thin plastic membrane of high porosity (several million pores per square inch) or a layer of sand. Such filter media retain suspended contaminants including bacteria and are used in various industrial and municipal applications. Clay suspensions are filtered by forced flow electrophoresis. Many fibrous or particulate materials often called "filter aids" can effect filtration; examples are special papers (Whatman), glass fibers, diatomaceous earth, fly ash, etc.
The construction and operations of many kinds of filtration equipments are too detailed to permit description. The most widely used filter types may be classified as follows (a) gravity filters used largely for water purification and consisting of thick beds of sand and gravel, which retain the flocculated impurities as the water passes through; (b) pressure filters of plate-and-frame or shell-and-leaf construction, which utilize filter cloths of coarse fabric as a separating medium; (c) vacuum or suction filters of the rotating drum or disk type used on thick sludges and slurries; (d) edge filters; and (e) diatomaceous earth filters. Gel filtration is a chromatographic technique involving separation at the molecular level
Of the above five types of filters, only gravity rapid sand filtration, gravity slow sand filtration, and diatomaceous earth filtration are widely used in large scale municipal water and wastewater treatments. Accordingly, the three filtration processes are described in greater detail including advantages and disadvantages.
The gravity rapid sand filtration process is the most commonly found filtration technique in the United States of America for treating surface water sources. It provides the greatest latitude for treating a wide range of raw water qualities and raw waters subject to rapid shifts in quality. It is correspondingly the most complex and requires the greatest amount of operating expertise and attention. The rapid sand filtration process polishes the water which is pretreated by coagulation and sedimentation basins In a rapid sand filter, the filter media is usually 24 to 30 inches in depth. The sand media can be composed of 1 to 3 grades of sand varying in size and density An effective grain size in the 0.35 to 0.55 mm range is required for the filter media. Rapid sand filtration, as the name implies, has a filtering rate considerably higher than slow sand filtration. Rapid sand rates typically operate at 2 to 3 gpm/sf (gallons per square foot) of filter surface or about 50 times higher than the 0.03 to 0.10 gpm/sf for slow sand filter Under some ideal raw water conditions, assuming optimized pre-treatment, filtration rates of 6 to 8 gpm/sf can be reached. Rapid sand filtration can effectively treat turbidities above 100 NTU and color up to 75 color units. The process can treat raw water with coliform levels greater than 20,000 per milliliter, significantly greater than gravity slow sand filtration and datomaceous earth filtration. Disadvantages of rapid sand filtration include high capital and operating costs, extensive operator training, and expertise in water treatment, water chemistry and microbiology. It also demands constant operator attention as opposed to the slow sand process.
The slow sand filtration process was developed over 100 years ago and remains a viable treatment process today. A slow sand filter plant is a biological process rather than a physical chemical one. It relies on the development of a biological layer to remove suspended contaminants including microorganisms. Since it relies on living microorganisms to aid in the sand filtration process, slow sand filters do not adapt themselves well to rapid changes in raw water quality. Extremes in turbidities or organic loading can also adversely affect performance by rapidly clogging the filter beds. In a typical slow sand filter operation, raw water is passed through the filter media composed of at least 30 inches of a well graded filter sand upon which the biological mass called a "schmutzdecke" is developed. Approximately two to three weeks is required to develop the "schmutzdecke" and acceptable filtered water quality. The slow sand filtration rate is very low, varying from 0.03 to 0.10 gpm/sf when compared with the much high flows of 2.0 gpm/sf for rapid sand filters. For this reason, a large slow sand filter bed with approximately 1,000 square feet of filter surface area is needed to produce 1 mgd (million gallons per day). Up to 1 NTU is reasonable for the filtered water in this process. The slow sand process because of its low capital and operating costs and less sophisticated operations is ideally suited to small communities. The process, however, cannot handle raw water turbidities much above 10 NTU nor appreciable algae or color content. Maximum raw water coliform levels should be below 800/100 ml. Colloidal clays in the raw water are also unacceptable. In summary slow sand filtration is a relatively simple process that does not require a highly technical staff to operate. Disadvantages include a need for an extensive amount of land and limited application to superior raw water quality.
The diatomaceous earth (DE) filtration process is an acceptable method for fulfilling the drinking water treatment requirements. Although a slightly more complex process than gravity slow sand filtration, it is nevertheless considerably simpler than the gravity rapid sand filtration process The primary removal mechanism in a DE filter, also called a "precoat filter", is straining through interstices created by diatomaceous media grains. Diatomaceous earth is actually crushed, classified, and processed silica obtained by mining fossilized diatoms, skeletons of microscopic marine organisms. The created mean media pore size is approximately 5 to 20 microns in diameter. The DE filtration process consists of three basic steps: precoating, filtering with body feed, and cleaning. The precoat is the application of the filter cake, an initial thick layer of 3 to 5 mm of diatomaceous earth, on a support membrane or septum. The cake is applied by creating a DE slurry in an auxiliary tank and forcing the slurry through the filter septum until the desired precoat thickness is obtained. At this point, the filtering operation can begin by passing raw water through the precoat filter cake. However, since the initial cake clogs rather quickly with particular, a constant feed of additional of the DE is needed to maintain the filter cake permeability This is accomplished by introducing a "body feed" which is a small, supplemental amount of a DE media slurry mixed into the raw water. The additional slurry is slowly added to the filter cake and thereby, maintaining the permeability of the filtering surface. When the head loss though the filter cake has increased to an undesirable level, the raw water/body feed influent which is stopped and the filter cake on the septum is removed and discarded. The typical filter loading rate is 1.5 gpm/sf. Filter runs can last from several hours up to two days depending on raw water characteristics, especially the particulate content. The greater body feed content is needed for a higher level of raw water particulates to prevent clogging of the filter cake. Generally, the DE filtration is only applicable for raw water sources that have turbidities less than 5 NTU, little sediment and algae. High levels of these undesirable contaminants can lead to "blinding" of the media or very rapid increases in head loss across the filter media and results in very short filter runs.
Other then state-of-the-art filtration technologies includes the following:
(a) oil-water separation, flotation, filtration and adsorption system, in which separate process reactors are connected in series for wastewater treatment; (b) mixing, flocculation, flotation and filtration system, in which four unit processes are incorporated into one package plant for water or wastewater treatment; (c) direct filtration system in which separate flocculation and rapid sand filtration are connected in series for treatment in water with low turbidity, color, algae, and other organic matter; (d) inline filtration system in which polymer is fed to precoat a rapid sand filtration bed for more efficient filtration of potable water with low turbidity, color, algae, and other organic matter; (e) upward moving inclined filter screen systems in which the endless filter screen used for filtration separation of large suspended matter from water, (f) preliminary filter, secondary filter, and reverse osmosis system in which separate filtration units are connected in series for more efficient water purification; and (g) automatic backwashing filter comprising: (1) particulate filter media confined between first and second screens for efficient filtration; and (2) rotatable backwashing pipes to permit automatic backwashing of filter media.
Such conventional water and wastewater filtration processes and apparatuses thereof are described in the U.S. Pat. No. 4,151,093 to Krofta, U.S. Pat. No. 4,377,485 to Krofta, U.S. Pat. No. 4,626,345 to Krofta, U.S. Pat. No. 4,626,346 to Hall, U.S. Pat. No. 4,673,494 to Krofta, U.S. Pat. No. 4,673,498 to Swinney et al, U.S. Pat. No. 4,673,500 to Hoofnagle et al and L. K. Wang, using Air Flotation and Filtration in color and Giardia removal. U.S. Department of Commerce, National Technical Information Service, Springfield, Va., USA. Technical Report No. PB89-158398/AS. October 1988. L. K. Wang and W. J. Mahoney. Treatment of Storm Run-off by Oil-Water Separation, Flotation, Filtration and Adsorption, Part A: Wastewater Treatment. Proceedings of the 44th Industrial Waste Conference, P. 655-666, May, 1989. L. K. Wang, M. H. S. Wang and W. J. Mahoney. Treatment of Storm Run-off by Oil-Water Separation, Flotation, Filtration and Adsorption: Part B: Waste sludge Management. Proceedings of the 44th Industrial Waste Conference, P. 667-673, May, 1989. L. K. Wang, M. H. S. Wang, Reduction of Color, Odor, Humic Acid and Toxic Substances by Adsorption, Flotation and Filtration, AIChe E Symposium on Design of Adsorption Systems for Pollution Control, 18 P., August, 1989.